Hemostasis, the physiological process of preventing excess blood loss by arresting flow via the formation of a hemostatic plug while maintaining blood in a fluid state within intact blood vessels, is maintained by tightly regulated interactions of the blood vessel wall, blood platelets, and blood plasma proteins. Under normal conditions there is a delicate balance between the individual components of the hemostatic system. Any disturbances in this balance, called the hemostatic potential, can result in either uncontrolled bleeding or formation of unwanted blood clots (thrombosis). Clinical assessment of clotting function has long been recognized to be important in management of surgical patients. Preoperatively, assessment of clotting function of a patient's blood is utilized as a predictor of risk of patient bleeding, allowing advanced preparation of blood components. Perioperative monitoring of clotting function of a patient's blood is also important because coagulopathies can be induced by hemodilution of procoagulants, fibrinogen and platelets, as a result of consumption of coagulation factors during surgical procedures, or cardiac procedures (e.g., cardiopulmonary bypass). Post-operative assessment of clotting function can also be crucial to a patient's successful recovery.
Coagulation is defined as transformation of a liquid or solution into a soft, semi-solid or solid mass. Blood naturally coagulates or clots to form a barrier when trauma or pathologic conditions cause vessel damage. There are two well-recognized coagulation pathways: the Contact Activation or thromboplastin-controlled pathway (formerly known as the extrinsic pathway) and the Tissue Factor or prothrombin/fibrinogen-controlled coagulation pathway (formerly known as the intrinsic pathway). Both the Contact Activation and Tissue Factor pathways result in the production of thrombin, a proteolytic enzyme which catalyzes the conversion of fibrinogen to fibrin.
Blood coagulation or clotting assays are principally used for screening or diagnosis and/or monitoring the hemostatic or coagulation status of a subject (e.g., a patient). There are many types of coagulation assays, including prothrombin time (PT), partial thromboplastin time (PTT) or activated partial thromboplastin time (APTT), fibrinogen assay, thrombin clotting time (TCT, TAT, or TT), activated clotting time (ACT). PT monitors the Contact Activation pathway of coagulation, and is useful for monitoring, e.g., antithrombotic therapy, for example, warfarin therapy. PTT or APTT detects factor changes in the Tissue Factor coagulation cascade (e.g., factors VIII, IX, XI, XII, other enzymes and factors), and is used primarily to monitor heparin therapy. Similarly, ACT evaluates the Tissue Factor pathways of coagulation and is useful for monitoring e.g., anticoagulation therapy, e.g., heparin therapy in situations where an APTT test cannot be performed, such as, for example if a patient was administered a high dose of heparin. TCT is not sensitive to deficiencies in either pathway, and measures a common pathway at the level of prothrombin to test for fibrinogen polymerization. The fibrinogen assay by the Clauss method (clotting method) utilizes activating levels of thrombin to initiate coagulation of a sample, and resulting coagulation time correlates with levels of fibrinogen in the sample.
The majority of coagulation assays for clinical assessment of patients are performed using the PT test. The PT test measures the activation of the Contact Activation coagulation pathway by addition of tissue thromboplastin. PT tests can be used for a number of different applications, including, for example, monitoring patients undergoing antithrombotic therapy (e.g., anticoagulant therapy) and assessing the status of a various clotting disorders including, e.g., acquired platelet function defect, congenital platelet function defects, congenital protein C or S deficiency, deep intracerebral hemorrhage, DIC (Disseminated intravascular coagulation), factor II deficiency, factor V deficiency, factor VII deficiency, factor X deficiency, hemolytic-uremic syndrome (HUS), hemophilia A, hemophilia B, hemorrhagic stroke, hepatic encephalopathy, hepatorenal syndrome; hypertensive intracerebral hemorrhage, idiopathic thrombocytopenic purpura (ITP), intracerebral hemorrhage, lobar intracerebral hemorrhage, placenta abruption, transient ischemic attack (TIA), and Wilson's disease.
Traditionally, coagulation parameters are determined by “wet chemistry” testing, wherein an aliquot of blood sample is mixed with one or more liquid coagulation reagents and the point of time at which the blood clots is detected. Results are indicated either directly (in seconds) or in the form of derived quantities such as ratio to a respective normal value (in percent). With respect to PT, common derived results for clotting indication include % Quick and the WHO standard, INR (International Normalized Ratio) values.
A number of various apparatuses and methods exist for measuring coagulation time of blood samples. Coagulation detection methods include detecting an increase in viscosity (viscosity detection method), detecting turbidity (turbidity detection method), and combined viscosity/turbidity detection methods. Other methods of coagulation detection employ multi-layered porous membranes impregnated with one or more coagulation reagents. Impregnated coagulation reagent(s) initiate coagulation of a sample (e.g., a predetermined blood volume), producing a detectable signal and the assays sometimes require predetermined blood volumes. Still other methods employ detection of oscillation of magnetic particles suspended in a reagent in a changing electric field, wherein oscillations change as a blood sample clots. Still other methods simply measure a change in light absorbance through a sample before and after a clotting reaction.
Most current methods have limitations which make them unsuitable or inconvenient for point of care testing or home use. Some require special blood sample preparation and handling or sophisticated equipment, making them suitable only for central laboratory facilities having qualified staff. Others, though possible for home use, are not cost effective for commercialization, or encounter implementation challenges (e.g., methods that require filtration of a sample through porous membranes pose wetting and uniform reagent impregnation difficulties).
Furthermore, besides cost and challenge of operation, a number of methods do not measure coagulation directly; and most tests do not measure coagulation without the use of an additive. Indirect measurement has been known to pose problems of accuracy in many samples. Other methods, while appearing to function well, can be limited to a narrow range of blood types, therapeutic windows, restricted by a long list of interfering factors or require large volumes of blood.
Thus, current blood coagulation tests are generally complex and the bulk of them are performed in a centralized clinical laboratory, at a clinic, or at a physician's office. Required visits to a clinic or a doctor's office on a regular basis to monitor anticoagulation therapy can be both inconvenient and expensive for a patient. Thus, there is a need for easy-to-use, compact, and portable instruments to facilitate use at “point of care” (POC) locations, within a surgical suite, or for a patient to monitor blood coagulation status at home.